Abstract:

Apparatus for cutting an elongate element (2) comprises two lateral
devices (6) for gripping the element on either side of a central section.
A third device (10) fits between the lateral devices for gripping the
central section, and together the gripping devices (6,10) define a common
axis along which the element to be cut is aligned. With the devices
installed on the element the lateral devices (6) are rotated relative to
the third device (10) to twist and shear the central section from the
element on either side.

Claims:

1. Apparatus for cutting an elongate element by removal of a section
therefrom, which apparatus comprises two lateral devices for gripping the
element at either end of said section; a central device for gripping said
section, the gripping devices being adapted to define a common axis
substantially aligned with the axis of the element when installed
thereon; and means for rotating the central device relative to the
lateral devices about said common axis when installed on a said element
to shear a said section therefrom at either end thereof.

2. Apparatus according to claim 1 wherein each gripping device comprises
at least two jaws for gripping a said element.

3. Apparatus according to claim 2 wherein the jaws have teeth for engaging
axially extending grooves on a said element.

5. Apparatus according to claim 1 wherein the lateral gripping devices are
mounted on a common structure.

6. Apparatus according to claim 1 wherein the central device forms annular
faces adapted to be in juxtaposition to complementary faces formed on the
lateral devices, all of which faces are substantially perpendicular to
said common axis, when installed on a said element.

7. Apparatus according to claim 6 wherein each lateral device forms a boss
with a said complementary face thereon, each boss being received in one
of two recesses in the central device, the base of which defines said
parallel faces, when installed on a said element.

9. Apparatus according to claim 1 including a mechanism for operating the
rotating means.

10. Apparatus according to claim 9 wherein the mechanism is selectively
attachable to the devices.

11. A method of cutting an elongate element by removal of a section
therefrom; comprising gripping the element at either end of the section;
separately gripping the section itself; and rotating the section relative
to and about the axis of the element at either end thereof to shear the
element at both ends of the section.

12. A method according to claim 11, using apparatus comprising one central
and two lateral gripping devices on said axis of said elongate element
respectively gripping the section and the element at either end of the
section, with arms extending from the devices in a direction
perpendicular to said axis, said section being rotated relative to the
element at either end of the section by reciprocal rotation of the arms
about said axis.

13. Apparatus for cutting an elongate element comprising two lateral
devices for gripping either end of a length of said element; a central
device for gripping a section of said element within said length, the
gripping devices defining a common axis to be aligned with the axis of
the element when installed thereon; and wherein annular faces on the
central device are in juxtaposition to complementary faces on the lateral
devices, all of which faces are substantially perpendicular to said
common axis, and means for rotating the central device relative to the
lateral devices about said common axis when installed on a said element
to shear a said section therefrom at either end thereof.

14. Apparatus according to claim 13 wherein the complementary face on each
lateral device forms a boss received in one of two recesses in the
central device, the bases of which bosses have parallel faces
perpendicular to the common axis.

Description:

[0001]This invention relates to a method and apparatus for cutting
elongate elements where there is limited access to a respective element,
and debris around the element at the point of cutting must be minimised.
The invention has application in many engineering fields where
complicated and delicate assemblies must be dismantled for example, and
in surgery where similar situations must be addressed.

[0002]Many mechanical structures use elongate elements extending between
component parts to fix the relative position or rotational orientation of
those components. They can be used to prevent adjacent bolts from
loosening by passing the element through diametral holes and/or fix the
spacing between two components. After time, the coupling of the elements
to the bolts or components can become locked, with the consequence that
the element cannot be readily removed. The element may be incapable of
removal for other means. It may be that the element interconnects more
than two bolts or components for example, and only the coupling with two
of them needs to be dismantled.

[0003]In circumstances such as those described above, an element extending
between two components can be broken using single or double bladed
cutters, or by sawing. However, both these techniques can be difficult in
regions where access is restricted, and where it is important that there
is minimal movement of the element being cut and the severed ends.

[0004]The present invention is a shear technique for cutting an elongate
element. Reference is directed in this respect to International Patent
Publication No: WO00/07512 (incorporated herein by reference) in which
telescopically engaging gripping devices twist an elongate element in
opposite directions to break the element at a shear plane defined axially
between the devices. According to the present invention, an elongate
element is cut by the removal of a section therefrom using shear cutting
techniques. In the method of the invention, the element is gripped at
either end of the section and the section itself is gripped separately.
The section is then rotated relative to and about the axis of the
elongate element at either end thereof to shear the element at shear
planes defined at the respective ends of the section.

[0005]Apparatus according to the invention comprises two lateral devices
for gripping the element at either end of the section to be removed, and
a central device for gripping the section itself. When installed on the
element, the gripping devices are adapted to define a common axis
substantially aligned with the axis of the element, and means are
provided for rotating the central element relative to the lateral devices
about that common axis. The two lateral devices are typically mounted on
a common structure, and with the central device form an integrated
assembly when installed on the element to be cut. Various means can be
provided for accomplishing the requisite relative rotation of the central
device relative to the lateral devices. Typically, such means will
include lever arms extending from the respective devices. It will be
appreciated that, particularly where space is very restricted, multiple
and/or reciprocal relative rotation of the section to be removed relative
to the other element sections may be required. It will also be
appreciated that considerable force may be required, and a geared
mechanism coupled to the gripping devices or to arms extending therefrom
to effect the requisite movement or movements can be provided for the
purpose.

[0006]In preferred embodiments of the invention, when installed on the
element to be cut, the central gripping device forms annular faces in
juxtaposition to complementary faces formed on the lateral devices, all
of which faces are substantially perpendicular to the common axis of the
elongate element. The shear planes at which the section is separated from
the remainder of the element are defined by the respective juxtaposed
faces. In a particularly preferred embodiment, each lateral devices forms
a boss which, when installed on the element to be cut, is received in one
of two oppositely facing recesses in the central device, the base of
which defines parallel faces in juxtaposition with the faces of the
bosses. The respective boss and recess-engagements define simple plane
bearings for the relatively rotating devices.

[0007]Apparatus according to the invention will not normally be used in
situations where an exposed length of an elongate element must be
removed. In those circumstances a device of the kind described in
International Patent Specification No: WO00/07512 can be used. Normally
then, the gripping devices in apparatus according to the present
invention must be designed to engage laterally rather than axially, with
the element to be cut. To facilitate such engagement, each device will
normally comprises at least two jaws for gripping the element and these
jaws may have teeth for engaging the exposed surface of the element to be
cut. As is described below, the element may have axially extending
grooves on its surface for receiving such teeth. Any combination of jaws
on a gripping device will normally comprise at least two pivotally
interconnected jaws, with a latch or other suitable locking mechanism. It
will be appreciated in this context that the particular apparatus
according to the invention will be designed for cutting elements of a
particular diameter. Different jaws can be designed to accommodate
different diameters. In many specialist applications it is more
appropriate to match the apparatus with the particular element diameter.

[0008]The invention will now be described by way of example and with
reference to the accompanying schematic drawings wherein:

[0009]FIG. 1 shows a perspective view of apparatus according to the
invention installed on an element to be cut;

[0010]FIG. 2 is a detailed view showing how the central and the lateral
gripping devices complement each other; and

[0011]FIG. 3 illustrates a gripping device engaging an element to be cut.

[0012]As shown in FIG. 1, an elongate element 2 extends between two
components 4 which are themselves part of a substrate device or
mechanism, not shown. Two lateral gripping devices 6 extend from a common
arm 8 to surround and engage the element 2. A central gripping device 10
fits between the lateral devices 6, and is connected to an arm 12. With
the gripping devices installed, the central device 10 is locked around a
section of the element 2 (not visible) between the two lateral devices
and when the arm 12 is rotated about the axis of the element, and of the
aligned lateral gripping devices, it proceeds to disrupt the structure of
the material of the element 2 between the central and the respective
lateral gripping devices, and extended or repeated relative rotational
movement of the gripping devices will eventually shear the element at two
planes at either end of the section gripped by the central device, and
the central gripping device can then be removed with that element
section.

[0013]FIG. 2 shows in a little more detail, the manner in which the
central and lateral gripping devices cooperate. As can be seen, with the
gripping devices closed the central device 10 defines a hole 14 with an
annular recess 16 at either end (only one is shown). Each of the lateral
devices 6 is formed with a boss 18 which, when all three devices are
installed on the element, fits neatly into the corresponding recess 16 in
the central device. The central device has a fixed jaw 20 and a movable
jaw 22 so that when the movable jaw 22 is fully opened, the central
device can be fitted between the lateral devices with the bosses 18
received in one half of the respective recess 16 in the central device,
before closure to complete the two recesses around the two bosses 18 on
the lateral devices.

[0014]Each of the lateral gripping devices shown is also formed with a
fixed jaw 24 and a movable jaw 26 so that the lateral devices can be
installed on the element to be cut before the central device 10 is
fitted. Once all three devices are fitted and the respective jaws locked
around the element 2, the relative rotation of the gripping devices can
be initiated, and eventually the element will break at the shear planes
defined by the bosses and recesses. The movable jaw 22 on the lateral
devices is then released and all three devices can then be withdrawn with
the removed element section still held by the central gripping device.

[0015]FIG. 3 illustrates a preferred mechanism by which the central and
lateral gripping devices can effectively engage an elongate element to be
cut. The same mechanism can be used on the central and the lateral
devices but for ease of understanding, parts will be identified with
reference to the central gripping device 10. The device has a fixed jaw
20 and a movable jaw 22. The movable jaw is pivotally attached to the
fixed jaw by a hinge 28. The fixed and movable jaws define an opening of
circular cross-section for receiving the element 2. The movable jaw
extends around one half of that circular cross-section and its distal end
engages the opposite side of the fixed jaw 20 by means of a latching
mechanism 30. Although the twisting forces that must be applied are
considerable, in order to simultaneously break the element at two
relatively close shear planes, these forces do not apply directly to
either the hinge 28 or the latch 30. The abutting surfaces of the
diametrically opposed ends of the respective jaws should ensure that the
forces on the hinge 28 and latch 30 are minimised.

[0016]As can be seen in FIG. 3, each of the fixed and movable jaws 20 and
22 is formed with a pair of teeth 32 which extend the axial length of the
hole defined by the jaws in their closed position. The element is formed
with complementary axially extending grooves 34, and the four teeth are
received in the respective four grooves of the element.

[0017]According to the invention elongate elements or rods are broken
along cross-sectional shear planes. When adjacent sections are twisted
relative to each other and sufficiently beyond the elastic limit for a
stainless steel rod, the elastic limit is reached after relative twisting
about a shear plane of around 45°. In order to break a stainless
steel rod using the apparatus and method of the invention, we have found
that three reciprocal rotations of around 60° is required. For an
elongate element or rod of titanium, which has high elasticity, a single
relative rotation of around 60° to 65° can be sufficient.
Where multiple movements are required in the same rotational sense, then
a ratchet mechanism can be employed to accomplish this. Such a mechanism
may be required in any event where space is particularly restricted.

[0018]As noted above, the technique of the invention has application in
many engineering and surgical fields. In surgical operations where bone
sections or individual bones (such as vertebra in the spine) must be
fixed relative to each other, this is typically accomplished using an
elongate element extending between components secured in the respective
bone or bone part. When the fixation mechanisms must be removed or
replaced, it is generally easier and safer to cut the connecting element
or rod to remove the fixation components, than to dismantle the mechanism
with the element or rod intact. The restricted access to the fixation
mechanism will be readily appreciated, as will the need to avoid
dissipation of debris that would be generated if the element or rod were
cut. Similar situations are created in other engineering fields,
particularly where corrosion between components and rods effectively
locks them together.